1. Field of the Invention
The present invention relates to an optical recording medium that optically records and reproduces information by the action of light, in particular, laser light such as semiconductor laser light. More particularly, it relates to a cardlike information recording medium (hereinafter "optical card") handy to carry.
2. Related Background Art
Recent years, as the society is progressively informationized, various information recording carriers or mediums and optical information recording and reproducing apparatus for optically recording and reproducing information, including optical discs, optical cards and optical tapes, have been proposed as a means for dealing with a great variety of information in a high speed and high efficiency. These information recording mediums include those which can detect information by converting the binary-coded information to a change in strength of reflected light which accompanies the presence or absence of pits.
As a recording medium that relies on changes in optical reflectance, what is called a heat mode recording medium, in which recording is carried out by irradiating a recording layer with an energy beam such as a laser beam in the form of a spot and changing in part the state of the recording layer. Recording mediums of this type do not require any development processing or the like after writing of information, and are called DRAW (direct read after write) media that can "read immediately after writing", by which high density recording can be performed and additional writing can be also performed.
Among such optical recording mediums, research recent years has been geared towards the productions of optical cards that are very handy to carry, that can hold recorded information in a large volume, and that also have a higher security than magnetic cards.
In optical information recording-reproducing apparatus, there is a system in which tracking tracks such as grooves or light-screening areas are previously formed at given intervals on the surface of a substrate of the medium for the purpose of tracking servo for writing and reading.
In this system, the tracking tracks play a role as a guide of a recording laser beam and/or a reproducing laser beam, and hence the laser beam can trace the information recording tracks with improved accuracy and also it is possible to achieve a higher access than conventional optical recording mediums that make use of a substrate having no tracking tracks.
FIG. 3 is a diagrammatical plan view showing a recording format of a conventional optical card. In FIG. 3, an optical card 11 is provided thereon with an information recording track 33 arranged in plurality. The respective information recording tracks are separated from one another through tracking tracks 32 for the tracking servo of reproducing light or recording light. The optical card 11 is also provided thereon with a recording region 34 having the information recording track in plurality, and the information is formed by the action of recording light in the form of pits 31 on the information recording tracks within this region. An arrow A indicates the directions in which the optical card 11 moves at the time of recording and reproducing operations.
FIG. 4 is a schematical view showing the constitution of a recording-reproducing apparatus for an optical card.
In FIG. 4, the optical card 11 is movable by means of a rotating mechanism 46 in the directions of an arrow A which are in parallel to track grooves 2 with respect to recording and/or reproducing beams 47. As for light from a light source 45 such as a semiconductor laser provided in an optical head 41, it is converged through a lens system 42 and shed on the information recording track 33 of the optical card. Thus, the recording light causes an optically detectable change on a recording layer 3 of the information recording track. The reproducing light reflects from the recording layer 3, again passes through the lens system 42, is separated from the light emitted from the light source by means of a beam splitter 43, and enters a photodetector 44, where an electrical signal corresponding with the recorded information is outputted. Once the reproduction of the information recorded in the information recording track 33 is completed as a result of the relative movement of the optical card 11 with respect to the reproducing beam, the optical head 41 moves in the direction (not shown) perpendicular to the tracks of the optical card and also performs tracking servo to detect an other information recording track 33. Subsequently, this optical card is driven in the reverse direction and thus the information is reproduced in the same way.
Incidentally, conventional optical cards are prepared, as shown in FIGS. 2A and 2B, by forming a recording layer 3 on the substrate surface that embraces a track groove forming region of a transparent substrate 5 having a track groove 2 serving as a tracking track, and laminating thereto a protective substrate 1 via an adhesive layer 4.
Information is recorded or reproduced by irradiation with a laser beam through the transparent substrate 5. Tracking by the laser beam is carried out by utilizing a difference in phase or amplitude of the light reflected from the track groove 2 corresponding to a tracking track and the information recording track area.
The recording light used in the recording of information on such an optical card is so selected as to be of such an intensity that, when it is shed on the recording layer 3 of the optical card, the recording layer undergoes reaction with the light to form the optically detectable changes (pits). As for the reproducing light, a laser beam is used which has an intensity weakened to such an extent that no changes occur in the recording layer 3 when the beam is shed on the recording layer 3.
Any conventional optical cards have a recording layer provided with an adhesive layer in close contact and hence the recording layer is restrained from the change in state caused by the recording light, so that they have a low recording sensitivity. Accordingly, in the process of recording on such optical cards, it has been required to use a laser beam with a high intensity or to lower the recording speed on the optical cards.
As for the instance in which the information recorded in an optical card is reproduced, it is preferable to increase the intensity of the reproducing light so that the S/N ratio of the signal can be improved, as disclosed in U.S. Pat. No. 4,189,735 (Japanese Patent Application Laid-open No. 54-126005).
Since, however, the information in the optical card is reproduced usually by causing the optical card to move alternately backward and forward with respect to a reproducing laser beam, there is a point of time at which the speed of movement relative to the reproducing light on the optical card decreases. The relative speed becomes zero with the reverse of the direction of movement of the optical card when the reproduction from one track of the information recording track has been completed and the next reproduction turns to another one of information recording tracks of the plurality as previously described. In conventional optical cards, the recording layer corresponding to the region irradiated with the reproducing light at this time, that is, the region 21 in which the movement of the optical card is stopped and reversed (hereinafter "stop-and-reverse region") is irradiated with the reproducing light for a long time. As a result, the problem has been caused such that the materials of the recording layer are deteriorated by light or heat to cause a decrease in reflectance or that this reproducing light tends to cause writing. For example, if the reflectance has decreased on the recording layer in the stop-and-reverse region, the tracking by the laser beam can not be carried out in this region, so that it becomes impossible to detect the track in or from which information is recorded or reproduced. In addition, the focusing of the reproducing light, which relies on the reflected light of the reproducing light, can not be carried out. Namely, the optical card having a decrease in reflectance in the stop-and-reverse region of the optical card no longer enables reproduction.
For this reason, it has been impossible in the conventional optical cards to so much increase the power of the reproducing laser beam and hence has been impossible to reproduce signals with a satisfactorily high S/N ratio.
In the meantime, as a means for solving such problems, the present applicant has disclosed in European Patent Publication No. 259151 a method in which an optical unrecordable member that can have a reflectance equal to that of the recording region is provided at a marginal area other than the recording region.
In this method, however, it is difficult to select the member that can maintain the same amount of reflected light between the recording region and the region other than that. There is also a disadvantage that faulty operations such as out-of-AT (auto-tracking) or out-of-AF (auto-focusing) tend to occur.
As commonly known, there is another method in which a stabilizing agent effective in preventing heat deterioration is added to the recording layer. In this method, however, although the decrease in reflectance in the stop-and-reverse region 21 can be prevented to a certain extent, the recording sensitivity in the recording region 34 may concurrently decrease to cause the problem that a difficulty may occur in achieving a higher recording speed.
The addition of the stabilizing agent also can not bring about so much improvement in the intensity of the reproducing light when the decrease in reflectance or the writing in the stop-and-reverse region is taken into account.
The present applicant has also disclosed in European Patent Publication No. 280531 a method in which a film cover layer is laminated to the recording region on an organic optical recording layer without interposing an adhesive layer between them, to improve the recording sensitivity in the recording region. According to this method, the region not provided with this film cover layer has a close-contact structure to bring about a decrease in recording sensitivity, so that the recording layer can be prevented from deteriorating due to the reproducing light in the stop-and-reverse region of the optical card. When, however, the intensity of the reproducing light is increased, this method is still unsatisfactory for preventing the deterioration of the recording layer in the stop-and-reverse region. In particular, a decrease in reflectance is caused by the heat accumulated by the reproducing light, because of the close contact of the adhesive layer having a low temperature diffusion rate.